I don't think I would "announce" that I am Catholic in any way, except through my behavior!

If you follow the teachings of Jesus and are faithful to Sunday Mass, people will figure out you're Catholic.

Now if you want to be even more public about it, you could wear a cross or appear in T-Shirts from your youth group perhaps, but I don't think that's necessary myself. I think the indirect, not in-your-face approach works better.

I often remember to pray before meals when I'm in public. That often includes the Sign of the Cross. That might be another way you could--if you wish--publicly demonstrate your faith.